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The Evander Blog

There’s nothing more unsightly than a misted up double glazed window, but did you know you could be paying more for your heating bill as your property becomes less insulated as a result?

I regularly get asked about misted up windows, so I’ve put together this guide to help to debunk some of the mysteries and solutions around misted up double glazing

Why does double glazing mist up?

Double glazing is formed from two panes of glass sealed together, with a gap left between them, to form a double glazed unit. The air trapped inside the window (or gas depending on the rating of double glazing you have fitted), creates an insulating barrier that keeps heat in.

It is only when the seal on the glass breaks, or fails as is known in the industry, that condensation gets into the window, resulting in a misted up window. The seal can break for a number of reasons, but is mainly due to the age of the window and how it was manufactured (the seals don’t last forever), how it was originally fitted and any chemical cleaning material used on the window overtime that might have damaged the seal.

But I have misting on the outside of the Window?

This phenomenon is a natural occurrence and not a fault in the glass or window, and is not due to your double glazed window failing as described above.

The situation occurs, mainly in Spring and Autumn, when the glass temperature falls to a low level at night, with the moisture of a heavy morning dew condensing against the cold surface of the glass.

As the latest windows we fit are more thermally efficient than in the past, this occurrence does happen a lot with newer windows. However, you do not need to change the glass or change anything as it is a phenomenon of nature.

What effect does it have on my energy bill?

The Energy Efficiency Agency states that the average household will save £165 a year on energy bills if you have B-Rated windows fitted. A B-Rated Window (or indeed A-Rated) is not dependent on the frame, but the glass and type of glass you have fitted.

With the frame misting up, you are loosing the insulation the original window provided and more heat will escape, ensuring your heating bill will be higher.

Do I need to change the window frame?

This is a common misconception. As a misted up window is due to the seal on the glass, which has nothing to do with the frame itself. Indeed, as long as the frame is OK, you simply just need to replace the glass unit. You can even upgrade the glass to A-Rated energy efficient to get a better insulation and lower your heating bills even further.

Does drilling and inserting warm air solve the problem?

The simple answer is no. With the window misting up due to the seal breaking, even if you drill the window and blow warm air into the window to clear the condensation, the seal on the window is still broken. Unless the seal is replaced, condensation will still get into the window. Having this type of repair may well appear to ‘fix’ the window for a few months, it will never truly fix the window and you will have to repeat the repair a number of times over the year to clear the misting of the window.

As the seal remain broken with this type of repair, you will also loose insulation and spend more on heating bills as the air trapped in the originally window is no longer there as the seal is broken.

What about repairing the unit myself?

You could look to repair the unit, but unless the process involves taking the unit apart and replacing the seal, you will still have the same problem. It may also become an issue when you come to sell the property, as unless you have FENSA certification for the windows, you may contravene UK building regulations in doing the work yourself.

If you use a reputably company, you should also get a five year guarantee on the replacement glass unit, so if it does break again, you can get a replacement for free.

My Recommendation

If your window suffers from misting and condensation between the two pains of glass, the middle bit, then you really should replace the glass unit to maintain an energy efficient home. There will always be ‘quick fixes’ on the market, but with a broken seal, the only way to repair it is to replace it and get a guarantee of workmanship to protect you should it fail in future. Don’t be fooled by some stating that you need a new window frame – you could even upgrade you home to A-grade energy efficiency rating just by changing the glass. But do make sure you always use a FENSA regulated window fitter do that you don’t contravene UK Building Regulations.